1. Field of the Invention
This invention relates to an apparatus and method for testing the continuity of optical fibers and, more particularly, to an apparatus and method for testing the continuity of two different optical fibers utilizing a single optical path.
2. Discussion
Optical fibers are used in a wide variety of applications. In most of these applications it is important to be able to test the continuity of the fiber. Optical fibers are often subject to stresses that can lead to breaks or misalignment of the fibers. This can result in a significant reduction in light throughput, which may cause failure of the particular system in which it is used. Consequently a number of methods of testing optical fibers have been developed. These include methods which simply introduce a light pulse at one end of the fiber and measure the optical throughput at the other end. In many systems, however, only one end of the optical fiber is accessible during testing, and a number of test systems capable of testing a fiber from a single end have been developed. These include systems utilizing optical time domain reflectometry. A system offering improved resolution over optical time domain reflectometry is described in related patent application Ser. No. 087,366, entitled "Laser Initiated Ordnance System Optical Fiber Continuity Test" by Richard Jacobs, and assigned to the same assignee as the present invention. That patent application discloses an optical fiber test system utilizing a dichroic surface mounted at the remote end of an optical fiber. The dichroic surface reflects light of the test wavelength and transmits the wavelength of the operable light signal. In that system, a test pulse is sent down the fiber and the amplitude of the light reflected by the dichroic surface indicates whether the fiber is defective or not.
Test systems utilizing dichroic reflectors are useful, for example, in laser initiated ordnance systems, also known as laser firing units. In laser firing units a primary laser is used to activate an explosive in a missile or other device. In one type of laser firing unit, stages in a missile may be fired by a high-energy laser pulse travelling through an optical fiber. Typically, a sequencer directs the light pulse to a series of individual or pairs of optical fibers.
Existing optical fiber continuity test systems are usually adequate when testing one fiber at a time. However, when an individual laser beam is transmitted down two fibers at a time to fire a pair of ordnance systems simultaneously, conventional test systems have been found to have some drawbacks. This is because, while such test systems can distinguish when both fibers are bad, the system is not always able to tell the difference between a return pulse from two good optical fibers and a return pulse from one good plus one bad optical fiber. This results from inherent limitations in the resolution of the detecting system. When the return pulse from a bad fiber is combined with the pulse from a good fiber, the amplitude of the return pulse is not always reduced sufficiently to permit detection of the bad fiber. This is especially true when the two fiber paths are unbalanced as is the case when there are different number of connectors in the two paths. Consequently, in systems where a single light beam is directed into two optical fibers simultaneously there has not been a satisfactory way to test the continuity of both fiber paths.